Process for operation of electric reduction furnaces



Nov. 10, 1970 H. KLEE E 3,539,695

PROCESS FOR OPERATION OF ELECTRIC REDUCTION FURNACES Filed Oct. 25, 1968 United States Patent Office 3,539,695 Patented Nov. 10, 1970 3,539,695 PROCESS FOR OPERATION OF ELECTRIC REDUCTION FURNACES Helmut Klee, Knapsack, near Cologne, Dieter Schorning, Bruhl-Pingsdorf, Georg Herget and Georg Strauss, Knapsack, near Cologne, and Hermann Niermann, Bruhl, near Cologne, Germany, assignors t Knapsack Aktiengesellschaft, Knapsack, near Cologne, Germany, a corporation of Germany Filed Oct. 25, 1968, Ser. No. 770,789

Claims priority, application Germany, Dec. 4, 1967,

Int. Cl. H05b 7/18 US. Cl. 13-34 3 Claims ABSTRACT OF THE DISCLOSURE Process for operating electric reduction furnaces with one or more electrodes, particularly symmetrical threephase furnaces, comprising varying the intensity I of power supplied to the furnace, the specific resistance p of material to undergo reaction in the furnace or the said intensity I and the said resistance p so that the ratio between (1) a constant K, which is set to equal twice the value for the shortest distance (in centimeters) between the electrode mid-vertical and the furnace inside wall, and (2) the product of current intensity I (in amperes) and the particular specific resistance p of the material to undergo reaction (in Q-cm.) is within the range between 4.0 and 5.0 [A- -fl- The present invention relates to a method of operating electric reduction furnaces, particularly symmetrical threephase furnaces, such as those which are used, for example, for the electrothermal production of phosphorus, carbide and ferrosilicon.

It is known that electric furnaces often await repair during their operation. This necessarily means prolonged stoppage periods which are occasioned by damages to the furnace lining, electrode breaks and similar phenomena.

In an attempt to minimize the impairment of the furnace inside walls, it is customary to use the rough formula according to which the electrical power input of a threephase furnace, expressed in horsepowers, shall be equal to the surface area of a square, expressed in square inches, to be laid around the electrodes. Attempts have also been made to define an optimum ratio for the diameter of the electrode pitch circle and that of the furnace inside wall; for example, in the case of phosphorus production furnaces, it has been attempted to base the calculation of the needed furnace diameter on the empirical formula: r=0.05-N+2[m], wherein r is the distance between the furnace center and the rounded furnace edges, expressed in meters, and N is the furnace power in megawatt (cf. Wotschke, Grundlagen des elektrischen Schmelzofens, Halle 1933; Curtis Journal of Electrochemical Society 100, 81 C (1953) and Schmidt, Chemische Technik, volume 17, 157 (1965).

Furnaces of improved power are more particularly subject to impairment. The reason for this is that the diameter of the reaction circle around the electrodes, which forms the actual melting zone, cannot be prevented by conventional steps, such as those described heretofore,

from becoming so large that damages to the furnace lining are the result.

Novel features and advantages of the present invention will become apparent to one skilled in the art from a reading of the following description in conjunction with the accompanying drawing in which the single figure is a lateral cross-sectional view of a three phase electric reduction furnace within which the process of this invention is practiced.

As shown in the drawing, electrodes 10, 11 and 12 are arranged symmetrically within furnace lining wall 13, installed within casing 21 of electric reduction furnace 22. Line 14 represents the shortest distance between inner furnace Wall 13 at point 15 and axis 16 of electrode 10. During operation of furnace 22 there result (depending on electrical current intensities flowing through electrodes 10, 11 and 12) such high temperatures that the material or burden situated in the interior of the furnace is liquid. This creates fusion reaction circles 17, 18 and 19 which flow together in the area of the electrodes. In particular, fusion reaction circle 17 surrounds electrode 10, fusion reaction circle 18 surrounds electrode 11 and fusion reaction circle 19 surrounds electrode 12.

Fusion reaction circle 19 surrounding, for example, electrode 12 should have a radius 14 otherwise referred to as K/2 which causes furnace wall 13 to be tangentially contacted by fusion circle 19 at point 15. If fusion circle 19 does not tangentially contact furnace wall 15, the available furnace capacity is not fully utilized. However, if the current intensity (which passes through electrode 12) is greater and creates a greater radius of fusion circle 19 than indicated by line 14, this would damage the furnace wall adjacent point 15. This last-mentioned operating condition should be avoided as much as possible because it causes repeated repairs and furnace shutdowns.

According to the invention, the electrode current is maintained at such magnitude that maintains the following mathematic expression within the indicated range:

K [cm.] I[A]- [S2-em.] 1

In this connection K represents diameter 20 or double the magnitude of radius 14. K is dimensioned in centimeters in the above expression. I is the electric current magnitude passing through electrodes 10, 11 and 12, which is dimensioned in the above expression in A (amperes). Finally, p represents the specific ohmic resistance of the liquid furnace material or burden, and has the dimension (2 cm. In inserting these three values K, I and p into the above-named mathematic expression, the range limits 4 and 5 are governed by dimensions A- -Q- The invention further consists of increasing only the voltage established at furnace 22 for further increase in capacity, while the current magnitude stated in the mathematical expression is maintained substantially constant. This relationship is significant because with a constant current magnitude, a change in furnace capacity can also be obtained by varying the specific ohmic resistance.

It has now unexpectedly been found that these disadvantageous phenomena can be obviated. To this end, it is necessary to operate an electric furnace with current of predetermined strength consistent with the furnace dimensions. The diameter of the melting zone, i.e. that of the liquid reaction zone downstream of the electrodes, has more particularly been found to be a function of the current strength rather than of the furnace power, under otherwise identical conditions:

TABLE 1 Diameter of Current strength melting zone in amperes Load in megawatts in meters It is known that the specific resistance p can be reduced by increasing the quantity of conductive reaction componentthis means a relative reduction of the non-conducting or ill-conducting componentsor to use the conducting reaction component in the form of larger particles.

Given the use of current whose strength has been determined in accordance with the equation K/l-p=4.0 to 5.0 [A -Sl it is very advantageous merely to increase the voltage applied to the furnace, in order to further increase the furnace power.

The following Table 3 shows that the present process is also applicable to furnaces other than phosphorus production furnaces.

TABLE 3 Phosphorus Carbide Ferrosilicon furnace furnace furnace Current intensity=l. 60,000 A 145,000 A. 30,000 A Specific mixed resis- 1 8-2.2- 0.52-0.55- -1.10- tance= 10- [Stem 10- [Qcm.] [0cm.] Diameter of melting -500 cm -320 cm. -140 cm.

zone= Reduced melting 4.0-4.6 [A- -Q- 4.0-4.2 [A- -n- 4.5 [A- -nl This fact enables the power of electric furnaces to be increased to a maximum by means of the following steps:

(1) Reduction of the current strength down to a value, where the reaction circles of the electrodes are just tangent to the furnace inside lining.

(2) Increasing the voltage to a maximum value, which is set by the temperature acceptable for the furnace gas and furnace cover.

This method of operating electric furnaces has been found to produce further, partially unexpected effects. The green electrode mass-consumption rate is reduced upon reduction of the current strength. When the green electrode mass-consumption rate is found to be higher than that which corresponds to the electrode consumption velocity in the furnace, it is occasioned by electrode breaks.

TAB LE 2 Electrode mass- Current strength consumption rate in amperes Load in megawatts (kg/100 kg. P)

When the voltage is increased, the electrodes will be found to immerge less deeply into the furnace. This means less thermal stress for the furnace base and improved electrode resistance to mechanical deformation.

The process of the present invention comprises more especially varying the strength or intensity I of power supplied to an electric reduction furnace and/ or the specific resistance p of material to undergo reaction in the furnace so that the ratio between (1) a constant K, which is set to equal twice the value for the shortestdistance between the electrode mid-vertical and the furnace inside wall, and (2) the product of current strength or intensity I and the particular specific resistance p of the material to undergo reaction is within the range of 4.0 to 5.0

The constant K is measured in cm., the current strength or intensity I is measured in amperes and the resistance p is measured in 0 cm. The quotient K/I'p is defined as reduced melting zone. A K/I- -ratio between 4.2 and 4.6 [A" -Q is most preferably used.

No appreciable damage to the lining of carbide and ferrosilicon production furnaces is likely to occur given the use of the above K/I- -ratio. It is furthermore advantageous, once a minimum value of 4.0 [A -Q has been reached, merely to increase the voltage for increasing the furnace power.

The following examples further illustrate the present invention.

EXAMPLE 1 The furnace was a symmetrical three-phase phosphorus production furnace, wherein the electrode center points were spaced about 2000 mm. from the furnace wall. The furnace was first operated under 25 megawatt load with current of 52,000 amperes, corresponding to a ratio of K/I- =3.85 [A- -tl- The lining of the furnace so operated was found to need repair a number of times. The same furnace was operated later in accordance with the present invention. The power was increased to 32 megawatts by increasing the potential to 420 volts and reducing the current strength down to 43,000 amperes, corresponding to a ratio of K/I- =4.6 [A -t2- The furnace lining could not be found to need repair.

EXAMPLE 2 The furnace was a symmetrical three-phase phosphorus production furnace, first operated with current of 65,000 amperes, for a furnace power of 51 megawatts. The furnace was then operated in accordance with the invention. The power was increased to 60 megawatts and the current strength was reduced to 60,000. Despite the power increase to 60 megawatt, the product-specific electrode consumption, expressed in kg. electrode mass per kg. of phosphorus produced, was found to have been reduced by more than 20%, as a result of the lower current strength. The electrode immersion length (into the furnace) was found to have been reduced from initially 2.8 m. to 2.4 to 2.5 meters. Electrode breaks did not occur.

We claim:

1. A process for operating an electric reduction furnace having an inside wall, a liquid burden disposed within said inside wall having a specific ohmic resistance designated p and dimensioned in 9 cm., one or more electrodes disposed symmetrically within said inside wall with the shortest distance between their axes and said inside wall designated K/Z and dimensioned in cm, an electrical current supplied to said furnace designated I and dimensioned in amperes (A), said process comprising the steps of placing said liquid burden in said furnace, supplying electrical current to said furnace and being characterized by supplying said electrical current I to said furnace throughout its operation of such value that it maintains the following electrical quantity in the indicated range (with the bracketed subject matter being dimensions):

K [0111.] -1 -1 -1, -1 Q 1 A o-cm. Q 1

2. The process of claim 1, characterized in that the electrode current is maintained at such an intensity that the following electrical quantity is maintained in the indicated range:

3. The process of claim 1, characterized in that for the further increase of the furnace capacity only the voltage established in the furnace is increased, While the current intensity, I, in the electrical quantity set forth in claim 1 is maintained substantially constant.

References Cited BERNARD A. GILHEANY, Primary Examiner R. N. ENVALL, JR., Assistant Examiner 

